Semiconductor device, active matrix substrate, and display device

ABSTRACT

A switching circuit (semiconductor device) ( 18 ) includes two switching units (SW 1  and SW 2 ), which are connected in series to each other, and two capacitances (CS 1  and CS 2 ), where one electrode of one of the capacitances is connected to the connecting section of the switching units (SW 1  and SW 2 ) and one electrode of the other capacitance is connected to one end of the switching units (SW 1  and SW 2 ). To the other electrodes of the capacitances (CS 1  and CS 2 ), signals having a constant voltage or signals having a same phase are supplied. A bottom gate electrode (light-shielding film) ( 22 ) is formed for the switching unit (SW 2 ).

TECHNICAL FIELD

The present invention relates to a semiconductor device equipped withswitching elements such as transistors, and also to an active matrixsubstrate and a display device using the semiconductor device.

BACKGROUND ART

In recent years, liquid crystal display devices, for example, which arethinner and lighter among other characteristics than conventionalcathode-ray tubes are in wide use as flat panel displays for liquidcrystal televisions, monitors, portable phones, and the like. Among suchliquid crystal display devices, those using an active matrix substratefor a liquid crystal panel serving as the display panel are known. Suchactive matrix substrates include a plurality of data wirings (sourceelectrode wirings) and a plurality of scan wirings (gate electrodewirings) arranged in a matrix, and pixels disposed near theintersections of the data wirings and the scan wirings, each pixelhaving a switching element such as a thin film transistor (hereinaftersimply referred to as “TFT”) and a pixel electrode connected to theswitching element.

In general, in the active matrix substrate described above, thin filmtransistors for peripheral circuits are integrally provided in additionto those used as switching elements for pixel driving. Further, foractive matrix substrates used in liquid crystal display devices equippedwith a touch panel or in liquid crystal display devices equipped withilluminance sensors (ambient sensors), it has been proposed tointegrally provide photodiodes (thin film diode: TFD) as optical sensorsin addition to thin film transistors for pixel driving and forperipheral circuits. Thus, active matrix substrates use semiconductordevices equipped with a plurality of thin film transistors andphotodiodes.

In recent years, to meet the demand for lower power consumption featureof the above-mentioned liquid crystal panels with built-in opticalsensors and liquid crystal panels with built-in pixel memories, forexample, a need for leakage current reduction of thin film transistors(transistors) of the semiconductor devices described above is beginningto be recognized. A known structure for suppressing the leakage currentof transistors employed in conventional semiconductor devices is the LDDstructure, where a low concentration impurity region (LDD region:Lightly Doped Drain) having a higher resistance than the source regionor the drain region is disposed at least either between the channelregion and the source region or between the channel region and the drainregion.

Also, as described in Patent Document 1 below, for example, in the caseof conventional semiconductor devices, it has been considered aspossible to connect a plurality of (two, for example) thin filmtransistors (switching units) in series, i.e., first and second thinfilm transistors, and to connect a storage capacitance (capacitance) tothe connecting section of the first and second thin film transistors toset the source-drain voltage of the second thin film transistor, towhich the liquid crystal capacitance of the liquid crystal panel isconnected, to almost 0V and to make the leakage current of the secondthin film transistor very small. Also, it has been considered that sucha conventional semiconductor device can suppress the pixel voltagefluctuation at the liquid crystal capacitance. Also, regarding thisconventional semiconductor device, it has been proposed to furthersuppress the fluctuation of the pixel voltage, which voltage is at oneend of the first and second thin film transistors, by connecting anotherstorage capacitance to the second thin film transistor in parallel tothe liquid crystal capacitance.

RELATED ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Patent Application Laid-Open Publication No.H4-251818

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

However, in a conventional semiconductor device such as described above,no light-shielding film was disposed for the second thin film transistor(switching unit). Therefore, this conventional semiconductor device hada problem that it could not reliably suppress the leakage current, andconsequently fluctuations in the pixel voltage could not be avoided.

Here, problems with the conventional semiconductor device are describedin detail with reference to FIG. 28.

FIG. 28 is a graph showing the relationship between the source-drainvoltage and the leakage current of a thin film transistor.

In FIG. 28, voltages Vds shown on the horizontal axis representsource-drain voltages of one of the thin film transistors connected inseries. Also in FIG. 28, curve 80 represents the relationship betweenthe voltage Vds and (light) leakage current Ioff when illumination light(4200 lux, for example) is projected from the backlight device providedin the liquid crystal display device. Further, curves 81 and 82represent the relationship between the voltage Vds and the leakagecurrent Ioff when the ambient temperature of the thin film transistor is40° C. and 60° C., respectively.

As apparent from curves 80, 81 and 82, when the source-drain voltage Vdsof each thin film transistor is below approx. 1V, the leakage currentIoff caused by the illumination light is larger than the leakage currentIoff caused by the elevated ambient temperature. Therefore, conventionalsemiconductor devices could not reliably suppress the leakage current.As a result, the conventional semiconductor device could not suppressthe fluctuation of the pixel voltage, which is at one end of the firstand second thin film transistors (switching units).

In consideration of the problems described above, the present inventionis aiming at providing a semiconductor device that can reliably suppressthe leakage current even when a plurality of switching units areconnected in series and a capacitance is connected to a connectingsection of the switching units, and that can suppress the voltagefluctuation at one end of the plurality of switching units. The presentinvention is also aiming at providing an active matrix substrate and adisplay device using such a semiconductor device.

Means for Solving the Problems

In order to achieve the objectives stated above, a semiconductor deviceaccording to the present invention includes a switching unit having atleast one switching element,

wherein a plurality of the switching units are connected in series toeach other,

wherein the semiconductor device further includes a plurality ofcapacitances, one electrode of each of the capacitances being connectedto corresponding one of connecting sections of the plurality ofswitching units or to one end of the plurality of switching units,

wherein signals having a constant voltage or signals having a same phaseare supplied to the other electrode of each of the plurality ofcapacitances, and

wherein the semiconductor device further includes a light-shielding filmformed at least for the switching unit, among the plurality of switchingunits, that has a capacitance disposed on both sides of the switchingunits.

In the semiconductor device configured as described above, signalshaving a constant voltage or signals having the same phase are suppliedto the other electrodes of the plurality of capacitances. Thus, voltagesbetween the plurality of switching units can be set to the same voltage,and therefore leakage current caused by the voltage difference betweenone end and the other end of the individual switching unit can beprevented from occurring. Also, in the plurality of switching units, alight-shielding film is formed for at least the switching unit having acapacitance on both sides. This way, leakage current caused by light canbe prevented from occurring. As a result, a semiconductor device that,unlike conventional examples, can reliably suppress the leakage currenteven when the plurality of the switching units are connected in seriesand a capacitance is connected to the connecting section of theswitching units, and can minimize the voltage fluctuation at one end ofthe plurality of switching units can be configured.

Also in the semiconductor device, preferably an MIS(Metal-Insulator-Semiconductor) type transistor is used as the switchingelement of the switching unit.

In this case, the configuration of the switching unit can be simplified,and a semiconductor device that is easy to manufacture can readily beconfigured.

Also in the above-mentioned semiconductor device, preferably a doublegate structure transistor including a semiconductor layer, and a topgate electrode and a bottom gate electrode disposed to sandwich thesemiconductor layer is used in the switching unit as the MIS-typetransistor.

In this case, the current drive force (ON current) of the switching unitcan easily be increased.

Also in the above-mentioned semiconductor device, the top gate electrodeand the bottom gate electrode of the double gate structure transistormay be electrically connected to each other.

In this case, by controlling the potentials at the top gate electrodeand at the bottom gate electrode, the potential fluctuation at thebottom gate electrode caused by the capacitive coupling between thebottom gate electrode and the semiconductor layer can be prevented, andtherefore leakage current can be prevented from occurring.

Also in the above-mentioned semiconductor device, the top gate electrodeand the bottom gate electrode in the double gate structure transistormay be capacitively coupled with each other.

In this case, by controlling the potential at the top gate electrode,the potential at the bottom gate electrode can appropriately becontrolled, and the fluctuation of the potential at the bottom gateelectrode to an inappropriate level that is likely to trigger theleakage current, which is caused by the capacitive coupling between thebottom gate electrode and the semiconductor layer, can be prevented, andthus the leakage current can be prevented from occurring. In stead ofthis configuration, the potential at the capacitively coupled top gateelectrode may be controlled by controlling the potential at the bottomgate electrode.

Also in the above-mentioned semiconductor device, preferably the bottomgate electrode is used as the light-shielding film.

In this case, the configuration of the semiconductor device is preventedfrom becoming complex and large, and a semiconductor device that is easyto manufacture can readily be configured.

Also in the above-mentioned semiconductor device, the plurality ofcapacitances are preferably connected in parallel to each other.

In this case, the area of each of the plurality of capacitances can bereduced, and therefore a compact semiconductor device can easily beconfigured.

Also in the above-mentioned semiconductor device, the plurality ofswitching units are constituted of the first and the second switchingunits, which are connected in series, and when the capacitance value ofthe first capacitance connected between the first and the secondswitching units is Cs1, the capacitance value of the second switchingunit connected on the side opposite from the first switching unit isCs2, and the OFF leakage current value Ioff of the second switching unitis approximated by Equation (1) below, the capacitance ratios R1 and R2of the first and the second capacitances may satisfy Equation (2) andEquation (3) below, respectively.Ioff=Io×Vds ^(n)  (1)R1={n/(n+1)}×{(C+Cv)/C}±0.2  (2)R2={1/(n+1)}×{(C−n×Cv)/C}±0.2  (3)

where Vds is the voltage between one end and the other end of the secondswitching unit (0≦Vds≦1); Io is the leakage current when Vds=1(V); n=0.7to 0.8; Cs1:Cs2=R1:R2; R1+R2=1; Cs1+Cs2=C; and Cv is a capacitance valueof an external capacitance connected in parallel to the secondcapacitance for the second switching unit.

In this case, when the capacitance ratios R1 and R2 of the first andsecond capacitances are optimum, the voltage fluctuation at one end ofthe plurality of switching units can reliably be suppressed.

Also in the above-mentioned semiconductor device, the plurality ofswitching units are constituted of the first, the second, and the thirdswitching units that are connected in series, and when the capacitancevalue of the first capacitance connected between the first and thesecond switching units is Cs1, the capacitance value connected betweenthe second and the third switching units is Cs2, the capacitance valueof the third capacitance connected to an end of the third switching uniton the side opposite from the second switching unit is Cs3, and the OFFleakage current value Ioff of the second and the third switching unitsis approximated in Equation (1) below, the capacitance ratios R1, R2,and R3 of the first, then the second, and the third capacitances maysatisfy Equation (4), Equation (5), and Equation (6) below,respectively.Ioff=Io×Vds ^(n)  (1)R1={n×n/(n×n+n+1)}×{(C+Cv)/C}±0.15  (4)R2={n/(n×n+n+1)}×{(C+Cv)/C}±0.15  (5)R3={1/(n×n+n+1)}×{(C−n×n×Cv−n×Cv)/C}±0.10  (6)

where Vds is the voltage (0≦Vds≦1) between one end and the other end ofthe second and the third switching units; Io is the leakage current whenVds=1(V); n=0.7 to 0.8; Cs1:Cs2:Cs3=R1:R2:R3; R1+R2+R3=1; Cs1+Cs2+Cs3=C;and Cv is the capacitance value of the external capacitance connected tothe third switching unit, in parallel to the third capacitance.

In this case, when the capacitance ratios R1, R2, and R3 of the first,second, and third capacitances are optimum, the voltage fluctuation onone end of the plurality of switching units can reliably be suppressed.

Also in the above-mentioned semiconductor device, when the plurality ofswitching units are constituted of the first, second, and thirdswitching units connected in series, and when the capacitance value ofthe first capacitance connected between the first and second switchingunits is Cs1, the capacitance value of the second capacitance connectedbetween the second and third switching units is Cs2, the capacitancevalue of the third capacitance connected to an end of the thirdswitching unit on the side opposite to the second switching unit is Cs3,and the OFF leakage current Ioff of the second and third switching unitsis approximated by Equation (1) below, then, the capacitance ratios R1and R2 of the first and the second capacitances may satisfy Equation (7)and Equation (8) below, respectively.Ioff=Io×Vds ^(n)  (1)R1={n/(n+1)}×{(C−Cs3)/C}±0.1  (7)R2={1/(n+1)}×{(C−Cs3)/C}±0.1  (8)

where Vds is the voltage (0≦Vds≦1) between one end and the other end ofthe second and the third switching units; Io is the leakage current whenVds=1(V); n=0.7 to 0.8; and Cs1:Cs2=R1:R2; and Cs1+Cs2+Cs3=C.

In this case, even when the capacitance value Cs3 of the thirdcapacitance is set to a prescribed value, by setting the capacitanceratios R1 and R2 of the first and the second capacitances to optimumvalues, voltage fluctuations at one end of the plurality of switchingunits can reliably be suppressed.

Also in the above-mentioned semiconductor device, the plurality ofcapacitances may be split to be disposed on both sides of the pluralityof switching units, sandwiching the switching units.

In this case, compared to the case in which a plurality of capacitancesare formed on one side of the switching units, a more compactsemiconductor device can easily be configured.

An active matrix substrate of the present invention includes any one ofthe semiconductor devices described above.

The active matrix substrate configured as described above uses thesemiconductor device that can reliably suppress the leakage current evenwhen a plurality of switching units are connected in series and acapacitance is connected to the connection section of the switchingunits, and can suppress the voltage fluctuation at one end of theplurality of switching units. Therefore, the active matrix substratefeaturing a high performance and low power consumption can easily beconfigured.

A display device of the present invention includes a display unit fordisplaying information, and uses any one of the semiconductor devicesdescribed above.

The display device configured as described above uses the semiconductordevice that can reliably suppress the leakage current even when aplurality of switching units are connected in series and a capacitanceis connected to the connection section of the switching units, and cansuppress the voltage fluctuation at one end of the plurality ofswitching units. Therefore, the display device featuring a highperformance and low power consumption can easily be configured.

In the above-mentioned display device, a liquid crystal panel may beused as the display unit.

In this case, the pixel voltage fluctuation at the liquid crystalcapacitance of the liquid crystal panel, which liquid crystalcapacitance is provided at one end of the plurality of switching units,can be suppressed, and a liquid crystal display device featuring a highdisplay performance can easily be configured. Also, because the leakagecurrent can reliably be suppressed and the pixel voltage fluctuation canbe minimized, the frame frequency at the liquid crystal panel cansignificantly be lowered, and the power consumption of the liquidcrystal display device can easily be reduced.

Also in the above-mentioned display device, a reflective liquid crystalpanel may be used as the display unit.

In this case, the pixel voltage fluctuation at the liquid crystalcapacitance of the liquid crystal panel, which liquid crystalcapacitance is disposed at one end of the plurality of switching units,can be suppressed, and therefore, a liquid crystal display devicefeaturing a high display performance can easily be configured. Also,because the leakage current can reliably be suppressed to minimize thepixel voltage fluctuation, the frame frequency at liquid crystal panelcan significantly be lowered. Further, because a light-shielding filmfor blocking the light from the backlight device does not need to beinstalled, a liquid crystal display device featuring a simpleconfiguration and lower power consumption can easily be configured.

In the above-mentioned display device, preferably the reflective liquidcrystal panel includes polymer-dispersed liquid crystals in its liquidcrystal layer, and the reflective liquid crystal panel is aretroreflection type liquid crystal panel including a retroreflectionsheet.

In this case, a reflective liquid crystal display device using aretroreflection sheet and featuring a high display performance can beconfigured.

Preferably, the above-mentioned display device includes a sensor thatdetects the condition of the surrounding environment of the displayunit, and a display control unit to which image signals are inputted andwhich performs the drive control of the display unit, and

preferably the display control unit includes a frame frequencyadjustment unit that adjusts the frame frequency based at least oneither the detection results sent from the sensor or image signalsinputted.

In this case, the frame frequency adjustment unit can appropriatelyadjust the frame frequency of the display image displayed on the displayunit. Therefore, a display device with a high display performance caneasily be configured.

Also in the above-mentioned display device, the frame frequencyadjustment unit may adjust the frame frequency to or below a prescribedfrequency in accordance with the image signal inputted.

In this case, the power consumption of the display unit and the displaydevice can be reduced.

Effects of the Invention

The present invention can provide a semiconductor device that canreliably suppress the leakage current even when a plurality of switchingunits are connected in series and a capacitance is connected to theconnecting section of the switching units, and can minimize the voltagefluctuation at one end of the plurality switching units. The presentinvention can also provide an active matrix substrate and a displaydevice using such a semiconductor device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a liquid crystal display device according toEmbodiment 1 of the present invention.

FIG. 2 illustrates the configuration of the liquid crystal panel shownin FIG. 1.

FIG. 3 is a circuit diagram showing an equivalent circuit of theswitching circuit shown in FIG. 2.

FIG. 4 is a plan view showing the configuration of the main part of theabove-mentioned switching circuit.

FIG. 5( a), FIG. 5( b), and FIG. 5( c) are cross-sectional views takenalong the line Va-Va, line Vb-Vb, and line Vc-Vc, respectively, of FIG.4.

FIG. 6 is a graph showing the relationship between the optimumcapacitance ratio of the above-mentioned switching circuit and the pixelvoltage fluctuation.

FIG. 7 is a circuit diagram showing an equivalent circuit of a switchingcircuit according to Embodiment 2 of the present invention.

FIG. 8 is a plan view showing the configuration of the main part of theswitching circuit shown in FIG. 7.

FIG. 9( a), FIG. 9( b), and FIG. 9( c) are cross-sectional views takenalong the line IXa-IXa, line IXb-IXb, and line IXc-IXc, respectively, ofFIG. 8.

FIG. 10 is a circuit diagram showing an equivalent circuit of aswitching circuit according to Embodiment 3 of the present invention.

FIG. 11 is a plan view showing the configuration of the main part of theswitching circuit shown in FIG. 10.

FIG. 12( a), FIG. 12( b), and FIG. 12( c) are cross-sectional viewstaken along the line XIIa-XIIa, line XIIb-XIIb, and line XIIc-XIIc,respectively, of FIG. 11.

FIG. 13 is a circuit diagram showing an equivalent circuit of aswitching circuit according to Embodiment 4 of the present invention.

FIG. 14 is a plan view showing the configuration of the main part of theswitching circuit shown in FIG. 13.

FIG. 15( a), FIG. 15( b), and FIG. 15( c) are cross-sectional viewstaken along the line XVa-XVa, line XVb-XVb, and line XVc-XVc,respectively, of FIG. 14.

FIG. 16 is a plan view showing the configuration of the main part of aswitching circuit according to Embodiment 5 of the present invention.

FIG. 17( a), FIG. 17( b), and FIG. 17( c) are cross-sectional viewstaken along the line XVIIa-XVIIa, line XVIIb-XVIIb, and lineXVIIc-XVIIc, respectively, of FIG. 16.

FIG. 18 is a circuit diagram showing an equivalent circuit of aswitching circuit according to Embodiment 6 of the present invention.

FIG. 19 is a plan view showing the configuration of the main part of theswitching circuit shown in FIG. 18.

FIG. 20( a) and FIG. 20( b) are cross-sectional views taken along theline XXa-XXa and the line XXb-XXb, respectively, of FIG. 19.

FIG. 21 is a graph showing a relationship between the optimumcapacitance ratio and the pixel voltage fluctuation in the switchingcircuit shown in FIG. 18.

FIG. 22 is a graph showing another relationship between the optimumcapacitance ratio and the pixel voltage fluctuation in the switchingcircuit shown in FIG. 18.

FIG. 23 is a circuit diagram showing an equivalent circuit of aswitching circuit according to Embodiment 7 of the present invention.

FIG. 24 is a plan view showing the configuration of the main part of theswitching circuit shown in FIG. 23.

FIG. 25( a) and FIG. 25( b) are cross-sectional views taken along theline XXVa-XXVa and line XXVb-XXVb, respectively, of FIG. 19.

FIG. 26( a) and FIG. 26( b) illustrate a liquid crystal display deviceaccording to Embodiment 8 of the present invention when the voltage isON and when the voltage is OFF, respectively.

FIG. 27 is a plan view showing the configuration of the main part of aswitching circuit used in the liquid crystal display device shown inFIG. 26.

FIG. 28 is a graph showing the relationship between the source-drainvoltage and the leakage current of a thin film transistor.

DETAILED DESCRIPTION OF EMBODIMENTS

Below, preferred embodiments of a semiconductor device, an active matrixsubstrate, and a display device of the present invention are describedwith reference to figures. Described below are examples of cases inwhich the present invention is applied to the switching circuit for thepixel electrode used in the active matrix substrate of the liquidcrystal panel. Dimensions of constituting elements in the figures do notaccurately reflect the actual dimensions of the constituting elements,the actual ratio of dimensions of the constituting elements, or thelike.

Embodiment 1

FIG. 1 illustrates a liquid crystal display device according toEmbodiment 1 of the present invention. In FIG. 1, a liquid crystaldisplay device 1 of this embodiment includes a liquid crystal panel 2whose viewer's side (display surface side) is shown at the top of FIG.1, and a backlight device 3 disposed on the liquid crystal panel 2 onthe non-display side (shown at the bottom of FIG. 1) to illuminate theliquid crystal panel 2. The liquid crystal panel 2 constitutes a displayunit that displays the information.

The liquid crystal panel 2 includes a color filter substrate 4 and anactive matrix substrate 5 constituting a pair of substrates, andpolarizing plates 6 and 7 respectively provided on the color filtersubstrate 4 and the active matrix substrate 5 on the outer surfaces. Aliquid crystal layer, which is not shown, is held between the colorfilter substrate 4 and the active matrix substrate 5. Also, a flatplate-shaped transparent glass material or transparent synthetic resinsuch as acrylic resin is used for the color filter substrate 4 and theactive matrix substrate 5. A resin film such as TAC (triacetylcellulose)or PVA (polyvinyl alcohol) is used for the polarizing plates 6 and 7,which are respectively bonded to the color filter substrate 4 and theactive matrix substrate 5 to cover at least the effective display regionon the display surface of the liquid crystal panel 2.

The active matrix substrate 5 constitutes one of the pair of substrates.The active matrix substrate 5 has pixel electrodes, thin filmtransistors (TFT), and the like formed thereon between itself and theliquid crystal layer, which correspond to the plurality of pixelsincluded on the display surface of the liquid crystal panel 2 (describedin detail below). As described in detail below, the active matrixsubstrate 5 has a switching circuit (semiconductor device) of thepresent invention including the above-mentioned thin film transistor ona pixel by pixel basis. On the other hand, the color filter substrate 4constitutes another substrate of the pair of substrates. The colorfilter substrate 4 has a color filter, an opposite electrode, and thelike formed thereon between itself and the liquid crystal layer (notshown).

The liquid crystal panel 2 also has a FPC (Flexible Printed Circuit) 8connected to a control device (not shown) that controls the driving ofthe liquid crystal panel 2. The liquid crystal layer is driven on apixel by pixel basis to drive the display surface on a pixel by pixelbasis and display desired images on the display surface.

Any liquid crystal mode and any pixel structure can be employed for theliquid crystal panel 2. Also, any driving mode can be used for theliquid crystal panel 2. That is, the liquid crystal panel 2 can be anyliquid crystal panel that can display information. Therefore, in FIG. 1,the structure of the liquid crystal panel 2 is not shown in detail, anddescription of the structure is omitted.

The backlight device 3 includes a light-emitting diode 9 as a lightsource, and a light guide plate 10 disposed to face the light-emittingdiode 9. In the backlight device 3, the light-emitting diode 9 and thelight guide plate 10 a are held in place while the liquid crystal panel2 is disposed over the light guide plate 10 in a bezel 14 having anL-shaped cross section. The color filter substrate 4 has a case 11disposed thereon. Thus, the backlight device 3 is attached to the liquidcrystal panel 2, forming a transmissive liquid crystal display device 1in which illumination light from the backlight device 3 is projected tothe liquid crystal panel 2.

For the light guide plate 10, synthetic resin such as transparentacrylic resin, for example, is used, and light from a light-emittingdiode 9 enters the light guide plate 10. A reflective sheet 12 isdisposed on the light guide plate 10 on the side opposite from theliquid crystal panel 2 (on the opposite surface side). On the lightguide plate 10, on the side facing the liquid crystal panel 2(light-emitting surface side), optical sheets 13 composed of a lenssheet, a diffusion sheet, and the like are disposed. The optical sheets13 convert the light originated from the light-emitting diode 9 andguided to a prescribed light guide direction (left to right in FIG. 1)through inside the light guide plate 10 into planar illumination lighthaving a uniform luminance. The illumination light is then projected tothe liquid crystal panel 2.

Although a configuration using the backlight device 3, which is edge-littype and is equipped with the light guide plate 10, is described above,this embodiment is not limited to such. Alternatively, a direct lightingtype backlight device may be used. A backlight device having a lightsource other than the light-emitting diode, such as a cold cathodefluorescent tube, may also be used.

Next, the liquid crystal panel 2 is described in detail with referenceto FIG. 2 and FIG. 3.

FIG. 2 illustrates the configuration of the liquid crystal panel shownin FIG. 1. FIG. 3 is a circuit diagram showing an equivalent circuit ofthe switching circuit shown in FIG. 2.

In FIG. 2, the liquid crystal display device 1 (FIG. 1) includes a panelcontrol unit 15, which is the display control unit that controls thedriving of the liquid crystal panel 2 (FIG. 1) serving as the displayunit displaying information such as characters and images, and also asource driver 16 and a gate driver 17 that operate based on theinstruction signals from the panel control unit 15. The liquid crystaldisplay device 1 also includes a storage capacitance driver 21 thatoutputs a prescribed signal to a plurality of storage capacitancesdescribed below. Similar to the source driver 16 and the gate driver 17,the storage capacitance driver 21 operates based on the instructionsignals from the panel control unit 15.

The panel control unit 15 is disposed in the above-mentioned controldevice, and into which the image signal from outside the liquid crystaldisplay device 1 is inputted. Also, detection results sent from thethermal sensor TS detecting the ambient temperature of the liquidcrystal panel 2 and detection results sent from the optical sensor OSdetecting the intensity of the external light entering the liquidcrystal panel 2 are inputted to the panel control unit 15. The thermalsensor TS and the optical sensor OS constitute a sensor that detects thecondition of the surrounding environment of the liquid crystal panel(display unit) 2.

The panel control unit 15 includes an image processing unit 15 a thatperforms prescribed image processing on the image signal inputted andgenerates various instruction signals for the source driver 16 and thegate driver 17; a frame buffer 15 b that can store a frame worth ofdisplay data included in the image signal inputted; and a framefrequency adjustment unit 15 c that adjusts the frame frequency of thedisplay image displayed on the liquid crystal panel 2. The panel controlunit 15 performs the drive control of the source driver 16 and the gatedriver 17 according to the inputted image signal to display on theliquid crystal panel 2 the information according to the image signal.

The frame frequency adjustment unit 15 c is configured to adjust theframe frequency based on the detection results sent from thermal sensorTS and the optical sensor OS and on the inputted image signal (describedin detail below).

The source driver 16, gate driver 17, and storage capacitance driver 21are disposed on the active matrix substrate 5. Specifically, the sourcedriver 16 is disposed on the surface of the active matrix substrate 5outside the effective display region A of the liquid crystal panel 2 asthe display panel, along the horizontal direction of the liquid crystalpanel 2. The gate driver 17 is disposed on the surface of the activematrix substrate 5 outside the effective display region A, along thevertical direction of the liquid crystal panel 2. The storagecapacitance driver 21 is disposed on the surface of the active matrixsubstrate 5 outside the effective display region A to face the gatedriver 17, along the vertical direction of the liquid crystal panel 2.

The source driver 16 and the gate driver 17 are driver circuits thatdrive a plurality of pixels P provided on the side of the liquid crystalpanel 2 on a pixel by pixel basis. To the source driver 16 and the gatedriver 17, a plurality of source electrode wirings S1 to SM (M is aninteger of at least 2, hereinafter collectively referred to as “S”) anda plurality of gate electrode wirings G1 to GN (N is an integer of atleast 2, hereinafter collectively referred to as “G”) are connected,respectively. The source electrode wirings S and the gate electrodewirings G respectively constitute the data wiring and the scan wiring,and are arranged in a matrix such that they cross each other on atransparent glass material or a transparent synthetic resin member (notshown) included in the active matrix substrate 5. That is, the sourceelectrode wirings S are disposed on the above-mentioned member inparallel to the column direction of the matrix (in the verticaldirection of the liquid crystal panel 2), and the gate electrode wiringsG are disposed on the above-mentioned member in parallel to the rowdirection of the matrix (in the horizontal direction of the liquidcrystal panel 2).

On the active matrix substrate 5, a plurality of bottom gate electrodewirings G1′ to GN′ (N′ is an integer of at least 2, hereinaftercollectively referred to as “G′”) are disposed in parallel to theplurality of gate electrode wirings G1 to GN. Similar to the gateelectrode wirings G, the bottom gate electrode wirings G′ are connectedto the gate driver 17, and supplies a prescribed bottom gate signal tothe bottom gate electrode, which is described below. As the bottom gatesignal, a signal different from the scan signal (gate signal) suppliedto the gate electrode (to be described below) connected to the gateelectrode wiring G, for example, is used.

Further, on the active matrix substrate 5, a plurality of storagecapacitance common electrode wirings H1 to HP (P is an integer of atleast 2, hereinafter collectively referred to as “H”) are disposed inparallel to the plurality of gate electrode wirings G1 to GN. Thestorage capacitance common electrode wirings H are connected to thestorage capacitance driver 21, and is configured to supply to theelectrode (the other electrode) of each of the plurality of storagecapacitances prescribed signals such as signals having a constantvoltage or signals having a same phase.

In the proximity of the location where the source electrode wiring Sintersects with the gate electrode wiring G and the bottom gateelectrode wiring G′, a switching circuit 18 for the pixel electrode(i.e., for the pixel driver circuit) using the semiconductor device ofthe present invention, and the above-mentioned pixels P having the pixelelectrode 19 connected to the switching circuit 18 are disposed. At eachof the pixels P, the common electrode 20 is disposed facing the pixelelectrode 19, sandwiching the liquid crystal layer provided in theliquid crystal panel 2. That is, on the active matrix substrate 5, theswitching circuit 18, the pixel electrode 19, and the common electrode20 are disposed for each of the pixels.

As shown in FIG. 3, in the switching circuit 18, a plurality (two, forexample) of switching units, a first switching unit SW1 and a secondswitching unit SW2, are connected in series to one another. In the firstswitching unit SW1, two thin film transistors Tr1 a and Tr1 b areconnected in series to one another, and in the second switching unitSW2, two thin film transistors Tr2 a and Tr2 b are connected in seriesto one another. Each of the thin film transistors Tr1 a, Tr1 b, Tr2 a,and Tr2 b constitutes a switching element, and for each of the thin filmtransistors Tr1 a, Tr1 b, Tr2 a, and Tr2 b, a MIS(Metal-Insulator-Semiconductor) type transistor is used.

In the switching circuit 18, top gate electrodes of the thin filmtransistors Tr1 a, Tr1 b, Tr2 a, and Tr2 b, i.e., gate electrodes g1,g2, g3, and g4, are connected to the gate electrode wirings G. As thethin film transistors Tr2 a and Tr2 b of the second switching unit SW2,a double gate structure transistor including the top gate electrode(gate electrodes g3 and g4) and the bottom gate electrode 22 is used.The bottom gate electrode 22 is connected to the bottom gate electrodewiring G′. The bottom gate electrode 22 is configured into a single unitfor the two gate electrodes g3 and g4. Further, the bottom gateelectrode 22 also functions as a (lower) light-shielding film blockingthe illumination light from the backlight device 3 (described in detailbelow).

In the switching circuit 18, an upper light-shielding film, which isdescribed below, is provided to cover the first and the second switchingunits SW1 and SW2, so that light from outside the liquid crystal panel 2(external light) can be blocked as much as possible from entering thefirst and the second switching units SW1 and SW2.

Also, in the switching circuit 18, at the connecting section of thefirst and the second switching units SW1 and SW2, and at one end of theswitching circuit 18, the first and second capacitances, i.e., the firstand second storage capacitances CS1 and CS2 are connected, respectively.That is, one of the electrodes of the first storage capacitance CS1 isconnected to the connecting section of the first and second switchingunits SW1 and SW2. Also, one of the electrodes of the second storagecapacitance CS2 is connected to the second switching unit SW2 on theside opposite from the above-mentioned connecting section. The otherelectrodes of the first and second storage capacitance CS1 and CS2 areconnected to the storage capacitance common electrode wiring H. Thefirst and second storage capacitances CS1 and CS2 are connected inparallel to each other.

The source electrode and the drain electrode of the switching circuit 18are connected to the source electrode wiring S and to the pixelelectrode 19, respectively. Also, a liquid crystal capacitance CLC isformed between the pixel electrode 19 and the common electrode 20. Theliquid crystal capacitance CLC constitutes an external capacitance forthe second switching unit SW2 and is connected in parallel to the secondstorage capacitance CS2.

Further, the switching circuit 18 is set to provide the optimumcapacitance ratios of the first and second storage capacitance CS1 andCS2 in consideration of, among other factors, the size of the liquidcrystal capacitance CLC, as described below. The switching circuit 18 isalso configured to reliably suppress the voltage fluctuation at one endof the switching circuit 18, i.e., the pixel voltage fluctuation at theliquid crystal capacitance CLC.

Back in FIG. 2, on the active matrix substrate 5, in the respectiveregions defined by a matrix by the source electrode wiring S, and by thegate electrode wiring G and the bottom gate electrode wiring G′, aplurality of regions of the pixels P are formed. These pixels P includered (R) pixels, green (G) pixels, and blue (B) pixels. These RGB pixelsare arranged sequentially, for example, in this order, in parallel tothe respective gate electrode wirings G1 to GN. Further, the RGB pixelscan display corresponding colors through a color filter layer (notshown) provided on the color filter substrate 4.

On the active matrix substrate 5, a gate driver 17 sequentially outputsto the gate electrode wirings G1 to GN the scan signal (gate signal)that turns ON the gate electrodes g1 to g4 of the correspondingswitching circuit 18 based on the instruction signal from the imageprocessing unit 15 a. When the gate driver 17 sends the signal to thegate electrode wirings G1 to GN, it also sequentially outputs the bottomgate signal to the bottom gate electrode wirings G1′ to GN′, which pairwith the gate electrode wirings G1 to GN, for the bottom gate electrode22 of the corresponding switching circuit 18.

When the gate electrode wirings G1 to GN, which pair with the storagecapacitance common electrode wirings H1 to HP, receive their signal, thestorage capacitance driver 21 sequentially supplies a signal of aconstant voltage or signals of the same phase to the storage capacitancecommon electrode wirings H1 to HP based on the instruction signal fromthe image processing unit 15 a, i.e., for the other electrodes of thefirst storage capacitance CS1 and the second storage capacitance CS2 ofthe corresponding switching circuit 18.

The source driver 16 outputs the data signal (voltage signal (gradationvoltage)) according to the illumination of the display image (gradation)to the corresponding source electrode wirings S1 to SM based on theinstruction signal from the image processing unit 15 a.

The switching circuit 18 is described in detail below with reference toFIG. 4, FIG. 5( a), FIG. 5( b), and FIG. 5( c).

FIG. 4 is a plan view showing the configuration of the main part of theswitching circuit. FIG. 5( a), FIG. 5( b), and FIG. 5( c) arecross-sectional views taken along the line Va-Va, line Vb-Vb, and lineVc-Vc of FIG. 4, respectively.

As shown in FIG. 4, in the switching circuit 18, the silicon layer SL,which is a semiconductor layer configured into the shape ofapproximately straight line, is disposed under the gate electrodes g1 tog4 connected to the gate electrode wiring G. Also in the switchingcircuit 18, as shown in FIG. 4 with dotted lines, the bottom gateelectrode 22 is formed under the silicon layer SL. The bottom gateelectrode 22 is arranged to overlap the gate electrode wiring G in thevertical direction of FIG. 4 (direction of the thickness of the activematrix substrate 5), and includes the linear portion constituting thebottom gate electrode wiring G′ and a portion disposed under the gateelectrodes g3 and g4 of the thin film transistors Tr2 a and Tr2 b in thesecond switching unit SW2 and serving as a lower light-shielding filmthat shields the thin film transistors Tr2 a and Tr2 b from the light.

Also in the switching circuit 18, as shown in FIG. 4 with thedashed-dotted line, an upper light-shielding film 24 is formed over thesilicon layer SL. The upper light-shielding film 24 is disposed to coverthe thin film transistors Tr1 a, Tr1 b, Tr2 a, and Tr2 b of the firstand the second switching units SW1 and SW2. The upper light-shieldingfilm 24 is electrically connected to the gate electrode wiring G througha contact 23.

In the silicon layer SL, a low concentration impurity region (LDDregion: Lightly Doped Drain region) 45 for generating the first storagecapacitance CS1 and a low concentration impurity region 46 forgenerating the second storage capacitance CS2 are provided. In theactive matrix substrate 5, the low concentration impurity regions 45 and46 are disposed both under the storage capacitance common electrodewiring H to generate a prescribed storage capacitance.

As shown in FIG. 5( a) to FIG. 5( c), in the active matrix substrate 5,the switching circuit 18 is disposed for each pixel over the substratemain body 5 a made of a glass substrate. As shown in FIG. 5( a) and FIG.5( b), in the switching circuit 18, the bottom gate electrode 22 isformed on the substrate main body 5 a. Also in the switching circuit 18,a base insulating film 47 is formed to cover the bottom gate electrode22 and the substrate main body 5 a, and on the base insulating film 47,a silicon layer SL is provided. Also in the switching circuit 18, a gateinsulating film 48 is formed to cover the silicon layer SL and the baseinsulating film 47, and the gate electrodes g1 to g4 are formed on thegate insulating film 48.

On the switching circuit 18, the source electrode and the drainelectrode 44 formed in the source electrode wiring S are disposed on theinterlayer film 49 formed to cover the gate electrodes g1 to g4. Thesource electrode is connected to a source region 25 provided in thesilicon layer SL through a contact hole 42, and the drain electrode 44is connected to a drain region 41 provided in the silicon layer SLthrough a contact hole 43.

Also in the switching circuit 18, the thin film transistors Tr1 a, Tr1b, Tr2 a, and Tr2 b are N-type transistors. That is, in the siliconlayer SL, high concentration regions (indicated with the cross hatchingin FIGS. 5) 25, 29, 33, 37, and 41 into which N-type impurity such asphosphorus is implanted at a high concentration, low concentrationimpurity regions (LDD region, indicated in FIG. 5 with dots) 26, 28, 30,32, 34, 36, 38, 40, 45, and 46 into which N-type impurity is implantedat a low concentration, and channel regions 27, 31, 35, and 39respectively formed under the gate electrodes g1 to g4 are provided.

As shown in FIG. 5( b), in the switching circuit 18, the bottom gateelectrode 22 is disposed under the silicon layer SL, extending from themiddle point of the high concentration region 33 to the edge of thedrain region 41. That is, as described above, the bottom gate electrode22 is formed only under the thin film transistors Tr2 a and Tr2 b of thesecond switching unit SW2. Also, as described in detail below, thebottom gate electrode 22 is made of an opaque electrode material, andthe bottom gate electrode 22 is configured to double as a (lower)light-shielding film that prevents the light from the bottom side ofFIG. 5( b), such as the illumination light from the backlight device 3,from entering the low concentration impurity regions 34, 36, 38, and 40and the channel regions 35 and 39. Thus, in the switching circuit 18,leakage current due to the illumination light can be suppressed at thesecond switching unit SW2.

Further, in the switching circuit 18, the upper light-shielding film 24is provided on the interlayer film 49 such that it is in the same layerwith the source electrode and the drain electrode 44. As shown in FIG.5( b), the upper light-shielding film 24 is disposed between the sourceelectrode and the drain electrode 44, over the gate electrodes g1 to g4,to shield the low concentration impurity regions 26, 28, 30, 32, 34, 36,38, and 40 and the channel regions 27, 31, 35, and 39 from the light.That is, the upper light-shielding film 24 can prevent the light fromthe top side of the FIG. 5( b) from entering the low concentrationimpurity regions 26, 28, 30, 32, 34, 36, 38, and 40 and the channelregions 27, 31, 35, and 39.

Here, a method of manufacturing the switching circuit 18 is described indetail.

In FIG. 5( a) to FIG. 5( c), the bottom gate electrode 22 is formed bydepositing a metal such as molybdenum or tungsten on the substrate mainbody 5 a, and then conducting a patterning by photolithography andetching. Specifically, the film thickness of the bottom gate electrode22 is approx. 100 to 200 nm.

Next, the base insulating film 47 is formed by forming a SiN film and aSiO₂ film, for example, sequentially by CVD (Chemical Vapor Deposition)to the thickness of 100 nm each. Then, over the base insulating film 47,an amorphous silicon film is formed to the thickness of 50 nm, which isthen converted to a polysilicon through the laser crystallization. Next,boron is doped into the polysilicon as the channel dope for thresholdadjustment.

Next, over the polysilicon, a SiO₂ film is formed to the thickness of 80nm as a gate insulating film 48, and over the gate insulating film 48, afilm of metal such as molybdenum or tungsten is deposited and thenpatterned to form the gate electrodes g1 to g4. Using the gateelectrodes g1 to g4 as masks, N-type impurity, such as phosphorus isdoped at a low concentration to form low concentration impurity regions26, 28, 30, 32, 34, 36, 38, and 40. Next, a photoresist for securing theregions of the low concentration impurity regions 26, 28, 30, 32, 34,36, 38, and 40 (LDD length) is formed, and then phosphorus is doped toform the source region 25, drain region 41, and high concentrationregions 29, 33, and 37.

Here, the doping amount in the low concentration impurity regions 26,28, 30, 32, 34, 36, 38, and 40 is adjusted to obtain a sheet resistanceof about 50 kΩ to 150 kΩ (1×10¹³ to 10¹⁴/cm², for example). This dopingamount is doped to cancel out the P-type impurity (boron) doped earlierfor channel doping in forming the N-type low concentration impurityregions 26, 28, 30, 32, 34, 36, 38, and 40. In the source region 25,drain region 41, and the high concentration regions 29, 33, and 37,phosphorus is doped at about 1×10¹⁵/cm² to keep the sheet resistance tono more than 1 kΩ. Then, a heat treatment is conducted for 1 hour at500° C. to 600° C. to activate the impurity. Alternatively, for ashorter heat treatment time, heat treatment may be conducted for a fewminutes using a lamp annealing device at 650° C. to 700° C.

Next, a SiO₂ film and a SiN film are formed to a thickness of about 100nm to 300 nm each as an interlayer film 49. Contact holes 42 and 43 areformed for connection to the source electrode and the drain electrode44, respectively. The source electrode, the drain electrode 44, andmetal for wiring, i.e., Al or Al alloy, for example, or a film made bylaminating them are deposited and patterned.

Lastly, although not shown in the figures, to form a pixel electrode 19of the liquid crystal display device 1, a planarizing film made of resinor the like is formed after wirings are formed. On the planarizing film,a transparent electrode (ITO, for example) that will be the pixelelectrode 19 is formed. In some cases, a reflective electrode is formedof Al, Ag, or its alloys on the ITO.

In the description above, a formation method when the thin filmtransistors Tr1 a, Tr1 b, Tr2 a, and Tr2 b are constituted of N-typetransistors is described. However, when the thin film transistors Tr1 a,Tr1 b, Tr2 a, and Tr2 b are to be constituted of P-type transistors,P-type impurity such as boron is used to form the source region 25 andthe drain region 41. Also, because the driver circuits around the panelcan also be formed according to the formation method described above,the switching circuit 18 of this structure can be applied to a switchingelement or the like that is required to have a low leakage current.

Next, with reference to FIG. 6, reduction in the pixel voltagefluctuation as an effect of the optimization of the capacitance ratiosof the first and the second storage capacitance CS1 and CS2 in theswitching circuit 18 is described in detail.

FIG. 6 is a graph showing the relationship between the optimumcapacitance ratio and the pixel voltage fluctuation in the switchingcircuit.

In the switching circuit 18 according to this embodiment, thecapacitance ratios R1 and R2 of the first and the second storagecapacitances CS1 and CS2 are set to satisfy Equation (2) and Equation(3) below. Thus, in the switching circuit 18 according to thisembodiment, the capacitance ratios R1 and R2 of the first and secondstorage capacitances CS1 and CS2 are optimized, and therefore the pixelvoltage fluctuation can reliably be suppressed.

Specifically, the inventor of the present invention found that when thevoltage Vds between one end and the other end of the second switchingunit SW2 is between 0V and 1V, the leakage current Ioff of the secondswitching unit SW2 can be obtained from Equation (1) below. Further, theinventor of the present invention found that, based on an approximateequation of Equation (1), the capacitance values Cs1 and Cs2 of thefirst and the second storage capacitances CS1 and CS2 that minimize the(voltage) fluctuation ΔVpix of the pixel voltage Vpix, which is causedby the leakage current, can be expressed by Equation (A) and Equation(B) below. Further, the inventor of the present invention found that,based on Equation (A) and Equation (B), and the plots 70 in FIG. 6, theoptimum capacitance ratios R1 and R2 of the first and second storagecapacitances CS1 and CS2 can respectively be expressed in Equation (2)and Equation (3) below.Ioff=Io×Vds ^(n)  (1)Cs1=n/(n+1)×(C+Cv)  (A)Cs2=1/(n+1)×(C−n×Cv)  (B)R1={n/(n+1)}×{(C+Cv)/C}±0.2  (2)R2={1/(n+1)}×{(C−n×Cv)/C}±0.2  (3)

where Io is the leakage current when Vds=1(V); n=0.7 to 0.8;Cs1:Cs2=R1:R2; R1+R2=1; Cs1+Cs2=C; and Cv is the capacitance value ofthe external capacitance connected to the second switching unit SW2 inparallel with the second storage capacitance CS2, i.e., the capacitancevalue of the liquid crystal capacitance CLC.

When n=0.73, C=200(fF), Cv=100(fF) in Equation (A) and Equation (B),values subjected to ±0.2 in Equation (2) and Equation (3) above areR1=0.63 and R2=0.37, respectively, which are the optimum values.Further, from plots 70 showing the relationship between R1 and thefluctuation ΔVpix and the optimum condition AC (=0.63), the range withthe smallest increase in fluctuation ΔVpix was determined to be ±0.2 inEquation (2) and Equation (3).

Also, in the switching circuit 18 according to this embodiment, leakagecurrent at the first and second switching units SW1 and SW2 issignificantly reduced, and the fluctuation ΔVpix of the pixel voltageVpix becomes small, as indicated by plots 70. Therefore, in theswitching circuit 18 according to this embodiment, the frame frequencyadjustment unit 15 c can adjust the frame frequency to a prescribedfrequency (10 Hz, for example) or lower in accordance with the imagesignal inputted.

Specifically, according to the calculation by the inventor of thepresent invention, in the switching circuit 18 of this embodiment, whenn=0.73, C=200(fF), and Cv=100(fF), the time during which the fluctuationΔVpix is 10 mV (i.e., the time during which the voltage is maintained atthe liquid crystal capacitance CLC) is 437 ms. Therefore, in theswitching circuit 18 of this embodiment, the lowest frame frequency maybe set to 2.3 Hz. However, this frame frequency is the result of thecalculation based on the leakage current when the ambient temperature ofthe liquid crystal panel 2 (switching circuit 18) is 40° C. Therefore,in the actual switching circuit 18, the frame frequency adjustment unit15 c needs to set the minimum frame frequency in consideration of theincrease in leakage current due to the rise in the ambient temperatureand the stray light.

As described above, the frame frequency adjustment unit 15 c isconfigured to adjust the frame frequency based on the detection resultssent from the thermal sensor TS and the optical sensor OS, and the imagesignal inputted. That is, in the first and second switching units SW1and SW2, the leakage current fluctuates depending on the operationenvironment of the liquid crystal panel 2, i.e., the ambient temperatureand the external light. Therefore, the frame frequency adjustment unit15 c is configured to determine the leakage current of the first andsecond switching units SW1 and SW2 based on the detection results sentfrom the thermal sensor TS and the optical sensor OS, and is alsoconfigured to adjust the frame frequency so that the display images onthe liquid crystal panel 2 do not fluctuate. The frame frequencyadjustment unit 15 c is also configured to adjust the frame frequency tothe prescribed frequency or lower when the display image (image signal)is a still picture, and to adjust the frame frequency to the secondprescribed frequency (50 Hz, for example) or higher when the displayimage is a motion picture.

In the switching circuit (semiconductor device) 18 according to thisembodiment configured as described above, a signal of a constant voltageor signals of the same phase is supplied to the other electrodes of thefirst and second storage capacitances (capacitances) CS1 and CS2. Thus,voltages at the first and second switching units SW1 and SW2, which areconnected in series to each other, can be set to the same voltage, andthe leakage current caused by the voltage difference between one end andthe other end of the first switching unit SW1 and of the secondswitching unit SW2 can be prevented. Also, a bottom gate electrode(lower light-shielding film) 22 and an upper light-shielding film 24 areformed at least for the second switching unit SW2, among the firstswitching unit SW1 and at the second switching unit SW2, which has thestorage capacitances CS1 and CS2 connected on the respective sides. Thisway, light-induced leakage current can be prevented from occurring atthe second switching unit SW2. As a result, a switching circuit(semiconductor device) 18 can, unlike the conventional example describedabove, reliably suppress the leakage current even when a plurality ofswitching units are connected in series and a capacitance is connectedto the connecting section of the switching units, and can suppress thevoltage fluctuations at one end of the plurality of switching units.

In this embodiment, in the second switching unit SW2, the thin filmtransistors Tr2 a and Tr2 b are double-gate structure transistors.Therefore, according to this embodiment, the current drive force of thesecond switching unit SW2 (ON current) can easily be increased. Also,because the ON current can easily be increased, according to thisembodiment, the liquid crystal capacitance CLC can easily be chargedmore rapidly.

In this embodiment, because the first storage capacitance CS1 and thesecond storage capacitance CS2 are connected parallel with each other,the areas of the first storage capacitance CS1 and the second storagecapacitance CS2 can be reduced. Consequently, a compact switchingcircuit (semiconductor device) 18 can easily be configured.

In this embodiment, the switching circuit (semiconductor device) 18, inwhich the leakage current is reliably suppressed even when a pluralityof switching units are connected in series to one another and acapacitance is connected to a connecting section of the switching units,and the voltage fluctuation at one end of the plurality of switchingunits is suppressed, is used. As a result, an active matrix substrate 5and a liquid crystal display device (display device) 1 featuring a highperformance and low power consumption can easily be configured.

Also in this embodiment, because the liquid crystal panel 2 is used asthe display unit, the fluctuation in the pixel voltage at the liquidcrystal capacitance CLC of the liquid crystal panel 2, which is providedon one end of the switching circuit 18, a liquid crystal display device1 featuring a high display performance can easily be configured. Also inthis embodiment, because the leakage current can reliably be suppressedto reduce the pixel voltage fluctuation, the frame frequency at theliquid crystal panel 2 can significantly be reduced. Therefore, powerconsumption of the liquid crystal display device 1 can easily bereduced.

In this embodiment, because the frame frequency adjustment unit 15 c isconfigured to adjust the frame frequency based on the detection resultssent from the thermal sensor TS and the optical sensor OS and theinputted image signal, the frame frequency adjustment unit 15 c canappropriately adjust the frame frequency of display images displayed onthe liquid crystal panel (display unit) 2. As a result, a liquid crystaldisplay device 1 featuring a high display performance can easily beconfigured.

In this embodiment, because the frame frequency adjustment unit 15 cadjusts the frame frequency to a prescribed frequency or lower inaccordance with the image signal inputted, the power consumption of theliquid crystal panel (display unit) 2 and the liquid crystal displaydevice 1 can be reduced.

Embodiment 2

FIG. 7 is a circuit diagram showing an equivalent circuit of a switchingcircuit according to Embodiment 2 of the present invention. FIG. 8 is aplan view showing the configuration of the main part of the switchingcircuit shown in FIG. 7. FIG. 9( a), FIG. 9( b), and FIG. 9( c) arecross-sectional views taken along the line IXa-IXa, line IXb-IXb, andline IXc-IXc, respectively, of FIG. 8. As shown in the figures, the maindifference between this embodiment and Embodiment 1 described above isthat, in this embodiment, the thin film transistor (switching element)included in the first switching unit is a double gate structuretransistor. For elements common to Embodiment 1, same referencecharacters are used and redundant descriptions are omitted.

That is, as shown in FIG. 7, in the switching circuit 18 according tothis embodiment, thin film transistors Tr3 a and Tr3 b of the firstswitching unit SW1 are double gate structure transistors. Specifically,the thin film transistor Tr3 a and Tr3 b respectively include gateelectrodes g1 and g2, and also include a bottom gate electrode 22.

Also, as shown in FIG. 8, FIG. 9( a), FIG. 9( b), and FIG. 9( c), in theswitching circuit 18 according to this embodiment, the bottom gateelectrode 22 is essentially one electrode unitarily formed for the fourgate electrodes g1 to g4, and, like in Embodiment 1, it also functionsas the (lower) light-shielding film blocking the illumination light fromthe backlight device 3. That is, as shown in FIG. 9( b), the bottom gateelectrode 22 is formed under the silicon layer SL, between an edge ofthe source region 25 and an edge of the drain region 41, blocking thelight projected from the bottom side of FIG. 9( b), which is theillumination light from the backlight device 3, for example, fromentering the low concentration impurity regions 26, 28, 30, 32, 34, 36,38, and 40 and the channel regions 27, 31, 35, and 39.

With the configuration described above, this embodiment can provide thefunctions and effects similar to those of Embodiment 1. Also in thisembodiment, because all thin film transistors Tr3 a, Tr3 b, Tr2 a, andTr2 b of the first switching unit SW1 and the second switching unit SW2are double gate structure transistors, in the switching circuit(semiconductor device) 18 of this embodiment, the current drive force(ON current) can easily be increased and the charge time of the liquidcrystal capacitance CLC can more easily be reduced.

Embodiment 3

FIG. 10 is a circuit diagram showing an equivalent circuit of aswitching circuit according to Embodiment 3 of the present invention.FIG. 11 is a plan view showing the configuration of the main part of theswitching circuit shown in FIG. 10. FIG. 12( a), FIG. 12( b), and FIG.12( c) are cross-sectional views taken along the line XIIa-XIIa, lineXIIb-XIIb, and line XIIc-XIIc, respectively, of FIG. 11. As shown in thefigures, the main difference between this embodiment and Embodiment 2 isthat, in this embodiment, the number of thin film transistors (switchingelements) included in the second switching unit is one. For elementscommon to Embodiment 1 described above, same reference characters areused, and redundant descriptions are omitted.

That is, as shown in FIG. 10, in the switching circuit 18 of thisembodiment, one thin film transistor, i.e., a thin film transistor Tr2,is used in the second switching unit SW2. The thin film transistor Tr2is a double gate structure transistor, and includes a gate electrode g3and a bottom gate electrode 22.

As shown in FIG. 11, FIG. 12( a), FIG. 12( b), and FIG. 12( c), in theswitching circuit 18 of this embodiment, the bottom gate electrode 22 isessentially one electrode unitarily formed for the three gate electrodesg1 to g3, and, like in Embodiment 2, it functions as the (lower)light-shielding film blocking the illumination light from the backlightdevice 3. That is, as shown in FIG. 12( b), the bottom gate electrode 22is formed under the silicon layer SL, between an edge of the sourceregion 25 and an edge of the drain region 41, blocking the lightprojected from the bottom of the FIG. 12( b), which is the illuminationlight from the backlight device 3, for example, from entering the lowconcentration impurity regions 26, 28, 30, 32, 34, and 36 and thechannel regions 27, 31, and 35.

In the switching circuit 18 according to this embodiment, a highconcentration region 50 is disposed between the low concentrationimpurity region 36 and the drain region 41.

With the configuration described above, this embodiment can provide thefunctions and effects similar to those of Embodiment 2.

Embodiment 4

FIG. 13 is a circuit diagram showing an equivalent circuit of aswitching circuit according to Embodiment 4 of the present invention.FIG. 14 is the configuration of the main part of the switching circuitshown in FIG. 13. FIG. 15( a), FIG. 15( b), and FIG. 15( c) arecross-sectional views taken along the line XVa-XVa, line XVb-XVb, andline XVc-XVc, respectively, of FIG. 14. As shown in the figures, themain difference between this embodiment and Embodiment 1 is that, inthis embodiment, the (top) gate electrode and the bottom gate electrodeare electrically connected to each other. For elements common toEmbodiment 1 described above, same reference characters are used andredundant descriptions are omitted.

That is, as shown in FIG. 13, in the switching circuit 18 according tothis embodiment, the gate electrode wiring G and the bottom gateelectrode wiring G′ are electrically connected to each other. Thus, inthe switching circuit 18 of this embodiment, the gate electrodes (topgate electrodes) g1 to g4 and the bottom gate electrode 22 areelectrically connected to each other. As a result, the same gate signalis supplied to the gate electrodes g1 to g4 and to the bottom gateelectrode 22.

As shown in FIG. 14, FIG. 15( a), FIG. 15( b), and FIG. 15( c), in theswitching circuit 18 of this embodiment, the gate electrode wiring G iselectrically connected to the bottom gate electrode 22 (bottom gateelectrode wiring G′) through a contact 51.

With the configuration described above, this embodiment can provide thefunctions and effects similar to those of Embodiment 1. Also in thisembodiment, the gate electrodes g1 to g4 and the bottom gate electrode22 are electrically connected to each other. Therefore, by controllingthe potentials at gate electrodes g1 to g4, the potential at the bottomgate electrode 22 can appropriately be controlled. As a result,according to this embodiment, fluctuation of the potential at the bottomgate electrode 22 due to the capacitive coupling of the bottom gateelectrode 22 and the silicon layer (semiconductor layer) SL can beprevented, and consequently the leakage current can also be prevented.

Embodiment 5

FIG. 16 is a plan view showing the configuration of the main part of theswitching circuit according to Embodiment 5 of the present invention.FIG. 17( a), FIG. 17( b), and FIG. 17( c) are cross-sectional viewstaken along the line XVIIa-XVIIa, line XVIIb-XVIIb, and lineXVIIc-XVIIc, respectively, of FIG. 16. As shown in the figures, the maindifference between this embodiment and Embodiment 4 described above isthat, in this embodiment, two storage capacitance common electrodewirings, which are disposed in parallel to each other and electricallyconnected to each other, are employed. For elements common to Embodiment4, same reference characters are used and redundant descriptions areomitted.

That is, as shown in FIG. 16, FIG. 17( a), FIG. 17( b), and FIG. 17( c),the switching circuit 18 of this embodiment has two storage capacitancecommon electrode wirings H and H′. The storage capacitance commonelectrode wirings H and H′ are disposed such that they overlap with oneanother via low concentration regions 45 and 46 in the direction normalto the surface of FIG. 16. The storage capacitance common electrodewirings H and H′ are electrically connected to each other through acontact 52. Thus, in the switching circuit 18 of this embodiment, thefirst storage capacitance CS1 is composed of a low concentration region45, a portion of the gate insulating film 48 and a portion of thestorage capacitance common electrode wiring H located over the lowconcentration region 45, and a portion of the base insulating film 47and a portion of the storage capacitance common electrode wiring H′located under the low concentration region 45. The second storagecapacitance CS2 is composed of a low concentration region 46, a portionof the gate insulating film 48 and a portion of the storage capacitancecommon electrode wiring H located over the low concentration region 46,and a portion of the base insulating film 47 and a portion of thestorage capacitance common electrode wiring H′ under the lowconcentration region 46.

With the configuration described above, this embodiment can provide thefunctions and effects similar to those of Embodiment 4. Also in thisembodiment, because two storage capacitance common electrode wirings Hand H′ are provided, capacitance values of the first and the secondstorage capacitances can easily be increased.

Embodiment 6

FIG. 18 is a circuit diagram showing an equivalent circuit of aswitching circuit according to Embodiment 6 of the present invention.FIG. 19 is a plan view showing the configuration of the main part of theswitching circuit shown in FIG. 18. FIG. 20( a) and FIG. 20( b) arecross-sectional views taken along the line XXa-XXa and line XXb-XXb,respectively, of FIG. 19. As shown in the figures, the main differencebetween this embodiment and Embodiment 1 described above is that, inthis embodiment, three switching units are connected in series to oneanother, and the first to the third storage capacitances are provided.Another main difference is that, in this embodiment, the (top) gateelectrode and the bottom gate electrode are capacitively coupled. Forelements common to Embodiment 1, same reference characters are used andredundant descriptions are omitted.

That is, as shown in FIG. 18, in the switching circuit 18 according tothis embodiment, the first, the second, and the third switching unitsSW1, SW2, and SW3 are connected in series sequentially. For the first,the second, and the third switching units SW1, SW2, and SW3, thin filmtransistors Tr1, Tr2, and Tr3, each composed of a double gate structuretransistor, are used, respectively.

Also in the switching circuit 18 according to this embodiment, as shownin FIG. 19 and FIG. 20( a), the bottom gate electrode 22 is essentiallyone electrode unitarily configured for three gate electrodes g1 to g3,and, like in Embodiment 1, it also functions as the (lower)light-shielding film blocking the illumination light from the backlightdevice 3. That is, as shown in FIG. 20( a), the bottom gate electrode 22is formed below the silicon layer SL between an edge of the sourceregion 25 and an edge of the drain region 41, and prevents the lightfrom the bottom side of FIG. 20( a), i.e., the illumination light fromthe backlight device 3, for example, from entering the low concentrationimpurity regions 26, 28, 30, 32, 34, and 36 and the channel regions 27,31, and 35.

In the switching circuit 18 of this embodiment, as shown in FIG. 18, thefirst, the second, and the third storage capacitances CS1, CS2, and CS3are connected in parallel with each other. That is, one electrode of thefirst storage capacitance (the first capacitance) CS1 is connected tothe connecting section of the first and the second switching units SW1and SW2. One electrode of the second storage capacitance (secondcapacitance) CS2 is connected to the connecting section of the secondand the third switching units SW2 and SW3. Also, one electrode of thethird storage capacitance (the third capacitance) CS3 is connected tothe end of the third switching unit SW3 on the side opposite from theconnecting section with the second switching unit SW2. The otherelectrodes of the first, the second, and the third storage capacitancesCS1, CS2, and CS3 are connected to the storage capacitance commonelectrode wiring H. Also in this embodiment, the liquid crystalcapacitance CLC constitutes an external capacitance connected parallelto the third storage capacitance CS3 for the third switching unit SW3.

The first storage capacitance CS1 is, similar to that in Embodiment 1,composed of the low concentration region 45, and a portion of the gateinsulating film 48 and a portion of the storage capacitance commonelectrode wiring H located over the low concentration region 45. Thesecond storage capacitance CS2 is, similar to that of Embodiment 1,composed of the low concentration region 46, and a portion of the gateinsulating film 48 and a portion of the storage capacitance commonelectrode wiring H located over the low concentration region 46. Thethird storage capacitance CS3 is composed of the low concentrationregion 53, a portion of the gate insulating film 48 and a portion of thestorage capacitance common electrode wiring H located over the lowconcentration region 53.

Further, in the switching circuit 18 of this embodiment, the capacitanceratios of the first, the second, and the third storage capacitances CS1,CS2, and CS3 are set to the optimum value determined in consideration ofthe size of the liquid crystal capacitance CLC as described in detailbelow. Thus, the voltage fluctuation at the above-mentioned one end ofthe switching circuit 18, i.e., the pixel voltage fluctuation at theliquid crystal capacitance CLC, can be reliably suppressed.

In the switching circuit 18 of this embodiment, as indicated bycapacitance Cg in FIG. 18, the gate electrodes (top gate electrodes) g1to g3 and the bottom gate electrode 22 are capacitively coupled witheach other. That is, as shown in FIG. 20( b), the gate electrode wiringG and the bottom gate electrode 22 are disposed to overlap one anotherthrough the gate insulating film 48 and the base insulating film 47, andthe gate electrodes g1 to g3 and the bottom gate electrode 22 areconfigured to be capacitively coupled.

Next, with reference to FIG. 21, the reduction in the pixel voltagefluctuation as an effect of the optimization of the capacitance ratio ofthe first storage capacitance CS1, the second storage capacitance CS2,and the third storage capacitance CS3 in the switching circuit 18 isdescribed in detail.

FIG. 21 is a graph showing the relationship between the optimumcapacitance ratio and the pixel voltage fluctuation in the switchingcircuit shown in FIG. 18.

In the switching circuit 18 according to this embodiment, capacitanceratios of R1, R2, and R3 of the respective first, second, and thirdstorage capacitances CS1, CS2, and CS3 are set to satisfy Equation (4),Equation (5), and Equation (6) below. This way, in the switching circuit18 of this embodiment, capacitance ratios R1, R2, and R3 of the first,second, and third storage capacitances CS1, CS2, and CS3 are optimizedto reliably suppress the pixel voltage fluctuation.

More specifically, the inventor of the present invention found that whenthe voltages Vds between one end and the other end of the secondswitching unit SW2 and of the third switching unit SW3 are at least 0(V)and no more than 1(V), the leakage current Ioff of the second switchingunit SW2 and that of the third switching unit SW3 are obtained fromEquation (1) below. Further, the inventor of the present invention foundthat, based on an approximate equation of Equation (1), the capacitancevalues Cs1, Cs2, and Cs3 of the first, second and third storagecapacitances CS1, CS2, and CS3 that minimize the (voltage) fluctuationΔVpix of the pixel voltage Vpix which fluctuation is caused by theleakage current can be expressed by Equation (C), Equation (D), andEquation (E) below. Further, the inventor of the present invention foundthat, based on Equation (C), Equation (D), and Equation (E), and plots71, 72, 73, and 74 of FIG. 21, the optimum capacitance ratios R1, R2,and R3 of the first, the second and the third storage capacitances CS1,CS2, and CS3 can be expressed respectively by Equation (4), Equation(5), and Equation (6) below.Ioff=Io×Vdsn  (1)Cs1={n×n/(n×n+n+1)}×(C+Cv)  (C)Cs2={n/(n×n+n+1)}×(C+Cv)  (D)Cs3={1/(n×n+n+1)}×(C−n×n×Cv−n×Cv)  (E)R1={n×n/(n×n+n+1)}×{(C+Cv)/C}±0.15  (4)R2={n/(n×n+n+1)}×{(C+Cv)/C}±0.15  (5)R3={1/(n×n+n+1)}×{(C−n×n×Cv−n×Cv)/C}±0.10  (6)

where, Io is the leakage current when Vds=1(V); n=0.7 to 0.8;Cs1:Cs2:Cs3=R1:R2:R3; R1+R2+R3=1; Cs1+Cs2+Cs3=C; and Cv is thecapacitance value of the external capacitance connected in parallel tothe third storage capacitance CS3 for the third switching unit SW3,i.e., the capacitance value of the liquid crystal capacitance CLC.

When n=0.73, C=200(fF), and Cv=100(fF) in Equation (C), Equation (D),and Equation (E), values subjected to ±0.15 in Equation (4) and Equation(5), and the value subjected to ±0.10 in Equation (6) are R1=0.35,R2=0.48, and R3=0.16, respectively, which are optimum values. Further,the fluctuation ΔVpix when R3 is fixed to a particular value and theratio R1:R2 is changed is obtained by calculation. Specifically, therelationships between R1 and the fluctuation ΔVpix when R3 is fixed to0.05, 0.16, 0.25, and 0.35 are indicated as plots 71, 72, 73, and 74,respectively, in FIG. 21. Then, from plots 72 and the optimum conditionAC (=0.35), a range in which the increase of the fluctuation ΔVpix issmallest is determined to be ±0.15 in Equation (4) and Equation (5).Because the optimum value of R3 is 0.16, which is small, the toleranceof R3 is set to ±0.10.

Also in the switching circuit 18 of this embodiment, leakage current atthe first, second and third switching units SW1, SW2, and SW3 issignificantly reduced, and the fluctuation ΔVpix of the pixel voltageVpix becomes small as indicated by plots 72. Therefore, in the switchingcircuit 18 of this embodiment, the frame frequency adjustment unit 15 ccan, like in Embodiment 1, adjust the frame frequency to a prescribedfrequency (10 Hz, for example) or lower in accordance with the imagesignal inputted.

Specifically, according to the calculation by the inventor of thepresent invention, in the switching circuit 18 of this embodiment, whenn=0.73, C=200(fF), Cv=100(fF), the time during which the fluctuationΔVpix is 10 mV (i.e., the time during which the voltage is maintained atthe liquid crystal capacitance CLC) is 1017 ms. Therefore, in theswitching circuit 18 of this embodiment, the lowest frame frequency maybe set to 1.0 Hz. However, this frame frequency is the result of thecalculation based on the leakage current when the ambient temperature ofthe liquid crystal panel 2 (switching circuit 18) is 40° C. Therefore,in the actual switching circuit 18, the frame frequency adjustment unit15 c, like in Embodiment 1, needs to set the minimum frame frequency inconsideration of the increase in leakage current due to the rise in theambient temperature and stray light.

With the configuration described above, this embodiment can provide thefunctions and effects similar to those of Embodiment 1. Also in thisembodiment, because the gate electrodes g1 to g3 and the bottom gateelectrode 22 are capacitively coupled with each other, by controllingthe potentials at the gate electrodes g1 to g3, the potential at thebottom gate electrode 22 can appropriately be controlled. As a result,according to this embodiment, fluctuation of the potential at the bottomgate electrode 22 due to the capacitive coupling of the bottom gateelectrode 22 and the silicon layer (semiconductor layer) SL can beprevented, and consequently, the leakage current can also be prevented.

Modification Example of Embodiment 6

FIG. 22 is a graph showing another relationship between the optimumcapacitance ratio and the pixel voltage fluctuation at the switchingcircuit shown in FIG. 18. As shown in the figure, the main differencebetween this embodiment and Embodiment 6 described above is that, inthis embodiment, the capacitance ratios of the first and second storagecapacitances are determined using a fixed capacitance value of the thirdstorage capacitance. For elements common to Embodiment 6, same referencecharacters are used and redundant descriptions are omitted.

That is, when employing the switching circuit 18 of Embodiment 6 for theliquid crystal display device 1, from the perspective of reducing thefield through voltage at the liquid crystal display device 1, thecapacitance value Cs3 of the third storage capacitance CS3 is preferablyset to a value higher than the prescribed value. In other words, in theswitching circuit 18 of Embodiment 6, when the capacitance ratios R1,R2, and R3 for the first, the second, and the third storage capacitancesCS1, CS2, and CS3 are optimized, the areas of the first, second, andthird storage capacitances CS1, CS2, CS3 can become so large that theactual formation of the first, second, and third storage capacitancesCS1, CS2, and CS3 can be difficult. Therefore, the inventor of thepresent invention devised a way to reduce the fluctuation ΔVpix of thepixel voltage Vpix, i.e., by optimizing the capacitance ratios R1 and R2of the first and second storage capacitances CS1 and CS2 with thecapacitance value Cs3 of the third storage capacitance CS3 fixed to aprescribed value.

That is, in the switching circuit 18, capacitance ratios R1 and R2 ofthe first and second storage capacitances CS1 and CS2 are set to satisfyEquation (7) and Equation (8) below. Thus, in the switching circuit 18of this embodiment, the capacitance ratios R1 and R2 of the first andsecond storage capacitances CS1 and CS2 are optimized, and therefore,the pixel voltage fluctuation can reliably be suppressed.

Specifically, the inventor of the present invention found that when thevoltages Vds between one end and the other end of the second and of thethird switching units SW2 and SW3 are both between 0V and 1V, theleakage current Ioff of the second and third switching units SW2 and SW3can be obtained from Equation (1) below. Further, the inventor of thepresent invention found that, based on an approximate equation ofEquation (1), capacitance values Cs1, Cs2, and Cs3 of the first, second,and third storage capacitances CS1, CS2, and CS3 that minimize the(voltage) fluctuation ΔVpix of the pixel voltage Vpix, which fluctuationis caused by the leakage current, can respectively be expressed byEquation (F), Equation (G), and Equation (H) below. Further, theinventor of the present invention found that, based on Equation (F),Equation (G), and Equation (H) and plots 75 in FIG. 22, the optimumcapacitance ratios R1 and R2 of the first and second storagecapacitances CS1 and CS2 can respectively be expressed in Equation (7)and Equation (8) below.Ioff=Io×Vds ^(n)  (1)Cs1=n/(n+1)×(C−Cs3)  (F)Cs2=1/(n+1)×(C−Cs3)  (G)C=Cs1+Cs2+Cs3  (H)R1={n/(n+1)}×{(C−Cs3)/C}±0.1  (7)R2={1/(n+1)}×{(C−Cs3)/C}±0.1  (8)

where Io is the leakage current when Vds=1(V); n=0.7 to 0.8; andCs1:Cs2=R1:R2.

When n=0.73, C=200(fF), Cs3=100(fF), and Cv=100(fF) in Equation (C),Equation (D), and Equation (E), values subjected to ±0.1 in Equation (7)and Equation (8) above are R1=0.21 and R2=0.29, respectively, which arethe optimum values. Also, because R1+R2+R3=1 is satisfied, thecapacitance ratio R3 of the third storage capacitance CS3 when thecapacitance value Cs3 is fixed is 0.50. Further, from plots 75 showingthe relationship between R1 and the fluctuation ΔVpix, and the optimumcondition AC (=0.21), a range in which the increase in the fluctuationΔVpix is smallest, is determined to be ±0.1 in Equation (7) and Equation(8).

Also in the switching circuit 18 of this embodiment, leakage current atthe first, second, and third switching units SW1, SW2, and SW3 issignificantly reduced, and the fluctuation ΔVpix of the pixel voltageVpix becomes small as indicated by plots 75. Therefore, in the switchingcircuit 18 of this embodiment, the frame frequency adjustment unit 15 ccan, like in Embodiment 1, adjust the frame frequency to a prescribedfrequency (10 Hz, for example) or lower in accordance with the imagesignal inputted.

Specifically, according to the calculation by the inventor of thepresent invention, in the switching circuit 18 of this embodiment, whenn=0.73, C=200(fF), Cs3=100(fF), and Cv=100(fF), the time during whichthe fluctuation ΔVpix is 10 mV (i.e., the time during which the voltageis maintained at the liquid crystal capacitance CLC) is 458 ms.Therefore, in the switching circuit 18 of this embodiment, the lowestframe frequency may be set to 2.2 Hz. However, this frame frequency isthe result of the calculation based on the leakage current when theambient temperature of the liquid crystal panel 2 (switching circuit 18)is 40° C. Therefore, in the actual switching circuit 18, the framefrequency adjustment unit 15 c, like in Embodiment 1, needs to set theminimum frame frequency in consideration of the increase in leakagecurrent due to the rise in the ambient temperature and stray light.

With the configuration described above, this embodiment can provide thefunctions and effects similar to those of Embodiment 6. Also in thisembodiment, because the optimum capacitance ratios R1 and R2 of thefirst and second storage capacitances CS1 and CS2 are obtained with thecapacitance value Cs3 of the third storage capacitance CS3 fixed to aprescribed value, the field-through voltage of the liquid crystaldisplay device 1 can easily be reduced, and the first, second, and thirdstorage capacitances CS1, CS2, and CS3 can appropriately be formed.

Embodiment 7

FIG. 23 is a circuit diagram showing an equivalent circuit of aswitching circuit according to Embodiment 7 of the present invention.FIG. 24 is a plan view showing the configuration of the main part of theswitching circuit shown in FIG. 23. FIG. 25( a) and FIG. 25( b) arecross-sectional views taken along the line XXVa-XXVa and line XXVb-XXVb,respectively, of FIG. 19. As shown in the figures, the main differencebetween this embodiment and Embodiment 6 is that, in this embodiment,the first to third storage capacitances are split to be disposed on bothsides of the first to third switching units, sandwiching the switchingunits. For elements common to Embodiment 6, same reference charactersare used and redundant descriptions are omitted.

That is, as shown in FIG. 23, in the switching circuit 18 of thisembodiment, the first and the second storage capacitances CS1 and CS2are connected parallel to each other, and the third storage capacitanceCS3 is provided separately from the first and the second storagecapacitances CS1 and CS2. That is, one electrode of the first storagecapacitance (the first capacitance) CS1 is connected to the connectingsection of the first and the second switching units SW1 and SW2. Oneelectrode of the second storage capacitance (second capacitance) CS2 isconnected to the connecting section of the second and the thirdswitching units SW2 and SW3. Also, one electrode of the third storagecapacitance (third capacitance) CS3 is connected to the end of the thirdswitching unit SW3 on the side opposite from the connecting section withthe second switching unit SW2. The other electrodes of the first andsecond storage capacitances CS1 and CS2 are connected to the storagecapacitance common electrode wiring H, and the other electrode of thethird storage capacitance CS3 is connected to the storage capacitancecommon electrode wiring H″, which is configured to be independent of thestorage capacitance common electrode wiring H.

Also in the switching circuit 18 of this embodiment, as shown in FIG.24, FIG. 25( a), and FIG. 25( b), the storage capacitance commonelectrode wirings H and H″ are disposed to sandwich the gate electrodewiring G. Thus, in the switching circuit 18 of this embodiment, thefirst and the second storage capacitances CS1 and CS2 and the thirdstorage capacitance CS3 are formed to sandwich the first to the thirdswitching units SW1 to SW3.

Also, in the switching circuit 18 of this embodiment, because twostorage capacitance common electrode wirings H and H″ are used, there isno need to supply signals of the same voltage to the storage capacitancecommon electrode wirings H and H″. As long as the signals have the samephase, they can have different voltages.

With the configuration described above, this embodiment can provide thefunctions and effects similar to those of Embodiment 6. Also in thisembodiment, the first and the second storage capacitances CS1 and CS2and the third storage capacitance CS3 are formed to sandwich the firstto the third switching units SW1 to SW3. Therefore, compared to the casein which the first to the third storage capacitances CS1 to CS3 areformed on one side of the first to the third switching units SW1 to SW3,a more compact switching circuit 18 can easily be configured.

Embodiment 8

FIG. 26( a) and FIG. 26( b) illustrate the voltage ON/OFF states of aliquid crystal display device according to Embodiment 8 of the presentinvention. FIG. 27 is a plan view showing the configuration of the mainpart of the switching circuit used in a liquid crystal display deviceshown in FIG. 26. As shown in the figures, the main difference betweenthis embodiment and Embodiment 1 is that, in this embodiment, aretroreflection type liquid crystal panel including a retroreflectionsheet is used. For elements common to Embodiment 1, same referencecharacters are used and redundant descriptions are omitted.

That is, as shown in FIG. 26( a) and in FIG. 26( b), the liquid crystaldisplay device 1 of this embodiment includes a retroreflection typeliquid crystal panel 2′. In the liquid crystal panel 2′, a commonelectrode 20 and a horizontal alignment film 54 a are formed over thecolor filter substrate 4 in this order. On the active matrix substrate5, a retroreflection sheet 55, a pixel electrode 19, and a horizontalalignment film 54 b are disposed in this order. Between the horizontalalignment films 54 a and 54 b, a liquid crystal layer LC includingpolymer-dispersed liquid crystals 56 is provided. The polymer-dispersedliquid crystals 56 include liquid crystal molecules 56 a and polymerliquid crystal bases 56 b. Also in the polymer-dispersed liquid crystals56, only liquid crystal molecules 56 a respond to the electrical fieldand change their orientation.

Specifically, as shown in FIG. 26( a), when the voltage is ON, theliquid crystal molecules 56 a are aligned in the electrical fielddirection, and the liquid crystal bases 56 b do not change theirorientation. As a result, the liquid crystal layer LC is transparent.Therefore, the light projected from above the color filter substrate 4is refracted by the color filter substrate 4, the liquid crystal layerLC, and the like, reflected by the retroreflection sheet 55, refractedby the color filter substrate 4, the liquid crystal layer LC, and thelike, and then arrives at the vicinity of the eyes of the viewer. As aresult, only the light from the vicinity of the viewer's eyes isrecognized by the eyes of the viewer. This produces a black display.

On the other hand, as shown in FIG. 26( b), when the voltage is turnedOFF, the liquid crystal molecules 56 a and the liquid crystal bases 56 bare randomly oriented, and the liquid crystal layer LC is in a scatteredstate. Therefore, the light coming from above the color filter substrate4 is scattered by the liquid crystal layer LC, and then after beingreflected by the retroreflection sheet 55, is scattered by the liquidcrystal layer LC. As a result, most of the light returns to the viewer'sside. This produces a white display.

With the configuration described above, this embodiment can provide thefunctions and effects similar to those of Embodiment 1. Also in thisembodiment, because a reflective liquid crystal panel 2′ is used as thedisplay unit, the pixel voltage fluctuation at the liquid crystalcapacitance CLC of the liquid crystal panel 2′ provided on one end ofthe switching circuit 18 can be suppressed, and therefore a liquidcrystal display device 1 with a high display performance can easily beconfigured. Also in this embodiment, because the leakage current canreliably be suppressed to minimize the pixel voltage fluctuation, theframe frequency of the liquid crystal panel 2′ can be reducedsignificantly. Also, because no (lower) light-shielding film blockingthe light from the backlight device needs to be installed, a liquidcrystal display device with a simple configuration featuring low powerconsumption can easily be configured.

Also in this embodiment, because a liquid crystal layer LC includingpolymer-dispersed liquid crystals 56 and a retroreflection sheet 55 areused, a reflective liquid crystal display device 1 featuring a highdisplay performance using a retroreflection sheet 55 can be configured.

Other than the configuration described above, a configuration in whichthe switching circuit 18 according to any one of Embodiment 2 toEmbodiment 7 is used may also be applicable.

Embodiments described above are all examples and are not limiting thepresent invention in any way. The technological scope of the presentinvention is defined by the appended claims and all changes that comewithin the range of equivalency of the claims are intended to beembraced therein.

For example, in the description above, the present invention is appliedin the switching circuits for pixel electrodes used in the active matrixsubstrate of the liquid crystal display device. However, a semiconductordevice of the present invention includes a switching unit having atleast one switching element, and only needs to include a plurality ofswitching units connected in series to one another, and a plurality ofcapacitances, one electrode of each of the capacitances being connectedto the corresponding connecting section of the plurality of switchingunits or to one end of the plurality of switching units, and to supplysignals having a constant voltage or having a same phase to the otherelectrodes of the plurality of capacitances, and also to include alight-shielding film formed for at least the switching unit, among theplurality of switching units, having a capacitance disposed on bothsides of the switching unit. A semiconductor device of the presentinvention is not limited in any other way.

Specifically, the present invention can be applied to, for example,transflective type and reflective type liquid crystal panels, variousdisplay devices such as organic EL (Electronic Luminescence) elements,inorganic EL elements, and field emission displays, and active matrixsubstrates used in those display devices. A semiconductor device of thepresent invention can be applied to switching circuits used inperipheral circuits such as driver circuits, as well as to switchingcircuits for the pixel electrodes. The number of the switching unitsconnected in series is not limited to 2 to 3 as stated in descriptionsabove.

Also, cases in which one or two N-type transistors are used as theswitching elements of the switching unit are described above. However,the switching elements of the present invention are not limited to such.For example, a switching element may be composed of an N-type transistorand a P-type transistor connected in parallel to each other.

However, as in embodiments described above, preferably an MIS(Metal-Insulator-Semiconductor) type transistor is used as the switchingelement of the switching unit, because this type of transistor cansimplify the configuration of the switching unit, and therefore canreadily configure a semiconductor device that is easy to manufacture.

Described above are configurations in which the frame frequencyadjustment unit adjusts the frame frequency using both the detectionresults sent from the thermal sensor and the optical sensor and theimage signal inputted. However, the frame frequency adjustment unit ofthe present invention is not limited to this. Other configurations maybe employed as long as the frame frequency is adjusted based on at leasteither the detection results sent from the sensor that detects thecondition of the surrounding environment of the display unit or theimage signal inputted. That is, a frame frequency adjustment unit of thepresent invention may adjust the frame frequency appropriately bydetermining the level of leakage current in the switching circuit basedon the detection result sent from the thermal sensor on the ambienttemperature of the display unit, or the detection result sent from theoptical sensor on the external light to which the display unit isexposed, and may also adjust the frame frequency appropriately inaccordance with the display image displayed on the display unit.

Also, in the descriptions above, cases in which the upperlight-shielding film is made of a conductive body and is electricallyconnected to the gate electrode wiring are described. However, an upperlight-shielding film of the present invention is not limited to this.For example, instead of being electrically connected to the gateelectrode wiring, the upper light-shielding film may be floated or maybe constituted of a non-conductive body. However, if the upperlight-shielding film is formed in the same layer with the sourceelectrode and the drain electrode as in embodiments described above, theupper light-shielding film is preferably electrically connected to thegate electrode wiring.

Also in the description above, a low concentration impurity region (LDDregion) is used in the silicon layer (semiconductor layer) constitutingthe storage capacitance (capacitance). However, the capacitance of thepresent invention is not limited to this, and a channel region, forexample, may be used instead of the low concentration impurity region.

Also in the descriptions above, cases in which a top gate electrodestructure transistor is used are described. However, a bottom gateelectrode structure (reverse staggered structure) transistor may also beused, and in that case, the light-shielding film needs to be formed overthe transistor. Also, this transistor may be microcrystalline silicon oran amorphous silicon transistor as well as a polycrystalline silicontransistor.

Although cases in which the bottom gate electrode is utilized as the(lower) light-shielding film is described, the present invention is notlimited to this in any way. Specifically, the bottom gate electrode mayalso be configured using a transparent electrode, and a light-shieldingfilm may be disposed under the semiconductor layer and under the bottomgate electrode. In such a configuration, a light-shielding film made ofnon-conductive body may also be used.

However, as in embodiments described above, utilizing the bottom gateelectrode also as the light-shielding film is preferable in respect thatsuch a configuration more reliably prevents the structure of thesemiconductor device from becoming complex or oversized.

INDUSTRIAL APPLICABILITY

The present invention is useful for semiconductor devices that canreliably suppress the leakage current even when a plurality of switchingunits are connected in series and capacitances are connected toconnecting sections of the switching units, and can suppress the voltagefluctuation at one end of the plurality of switching units, and is alsouseful for active matrix substrates and display devices using such asemiconductor device.

DESCRIPTION OF REFERENCE CHARACTERS

1 liquid crystal display device (display device)

2, 2′ liquid crystal panel (display unit)

5 active matrix substrate

15 panel control unit (display control unit)

15 c frame frequency adjustment unit

18 switching circuit (semiconductor device)

22 bottom gate electrode ((lower) light-shielding film)

24 upper light-shielding film

55 retroreflection sheet

56 polymer-dispersed liquid crystal

SW1 first switching unit

SW2 second switching unit

SW3 third switching unit

CS1 first storage capacitance (first capacitance)

CS2 second storage capacitance (second capacitance)

CS3 third storage capacitance (third capacitance)

SL silicon layer (semiconductor layer)

Tr1 a, Tr1 b, Tr1, Tr2 a, Tr2 b, Tr2, Tr3 a, Tr3 b, Tr3 thin filmtransistor (switching element)

g1, g2, g3, g4 gate electrode (top gate electrode)

CLC liquid crystal capacitance (external capacitance)

TS thermal sensor (sensor)

OS optical sensor (sensor)

LC liquid crystal layer

The invention claimed is:
 1. A semiconductor device comprising a switching unit having at least one switching element, wherein a plurality of said switching units are connected in series to each other, wherein said semiconductor device further includes a plurality of capacitances, one electrode of each of said capacitances being connected to corresponding one of connecting sections of said plurality of switching units or to one end of said plurality of switching units, wherein signals having a constant voltage or signals having a same phase are supplied to the other electrode of each of said plurality of capacitances, wherein said semiconductor device further includes a light-shielding film formed for at least the switching unit, among said plurality of switching units, that has a capacitance disposed on both sides of said switching unit, wherein said plurality of switching units are constituted of a first, a second, and a third switching units which are connected in series, and wherein when a capacitance value of a first capacitance connected between said first and said second switching units is Cs1, a capacitance value of a second capacitance connected between said second and said third switching units is Cs2, a capacitance value of a third capacitance connected to an end of said third switching unit on the side opposite from said second switching unit is Cs3, and an OFF leakage current values Ioff of said second and said third switching units are approximated in Equation (1), capacitance ratios R1, R2, and R3 of said first, second, and third capacitances satisfy Equation (4), Equation (5), and Equation (6) below, respectively: Ioff=Io×Vds ^(n)  (1) R1={n×n/(n×n+n+1)}×{(C+Cv)/C}±0.15  (4) R2={n/(n×n+n+1)}×{(C+Cv)/C}±0.15  (5) R3={1/(n×n+n+1)}×{(C−n×n×Cv−n×Cv)/C}±0.10  (6), where Vds is a voltage between one end and the other end of said second switching unit and said third switching unit (0≦Vds≦1); Io is a leakage current when Vds=1(V); n=0.7 to 0.8; Cs1:Cs2:Cs3=R1:R2:R3; R1+R2+R3=1; Cs1+Cs2+Cs3=C; and Cv is a capacitance value of an external capacitance connected in parallel to said third capacitance for said third switching unit.
 2. The semiconductor device according to claim 1, wherein an MIS (Metal-Insulator-Semiconductor) type transistor is used as the switching element of said switching unit.
 3. The semiconductor device according to claim 2, wherein said switching unit uses a double gate structure transistor including a semiconductor layer, and a top gate electrode and a bottom gate electrode disposed to sandwich said semiconductor layer, as said MIS type transistor.
 4. The semiconductor device according to claim 3, wherein said top gate electrode and said bottom gate electrode are capacitively coupled with each other in said double gate structure transistor.
 5. The semiconductor device according to claim 3, wherein said bottom gate electrode is used as said light-shielding film.
 6. The semiconductor device according to claim 1, wherein said plurality of capacitances are connected in parallel to each other.
 7. The semiconductor device according to claim 1, wherein said plurality of capacitances are split to be formed on both sides of said plurality of switching units to sandwich said plurality of switching units.
 8. An active matrix substrate comprising the semiconductor device according to claim
 1. 9. A display device comprising a display unit that displays information and the semiconductor device according to claim
 1. 10. The display device according to claim 9, wherein said display unit is a liquid crystal panel.
 11. The display device according to claim 9, wherein said display unit is a reflective liquid crystal panel.
 12. The display device according to claim 11, wherein said reflective liquid crystal panel includes polymer-dispersed liquid crystals in its liquid crystal layer, and wherein said reflective liquid crystal panel is a retroreflection type liquid crystal panel including a retroreflection sheet.
 13. The display device according to claim 9, comprising a sensor detecting a condition of environment surrounding the display unit; and a display control unit to which image signals are inputted and which performs drive control of said display unit, wherein said display control unit includes a frame frequency adjustment unit that adjusts a frame frequency based at least on either detection results sent from said sensor or the image signals inputted.
 14. The display device according to claim 13, wherein said frame frequency adjustment unit adjusts the frame frequency to or below a prescribed frequency in accordance with the image signal inputted. 